In neutron spectrometry and especially time-of-flight spectrometry, use is generally made of rotating mechanical selector and chopper systems for the purpose of neutron energy selection. A nuclear reactor or other neutron source delivers a neutron beam having a wide energy spectrum, i.e. a wide range of neutron velocities. The chopper and mechanical selector system is intended to derive from the broad-spectrum beam of neutrons, a monochromatic beam, i.e. a beam of neutrons which are substantially monoenergetic or of the same velocity.
Monochromatization in this manner is usually done roughly by drum selectors in which the drum consists of a neutron-absorbent material provided with axial and helical grooves. At a fixed speed of the drum, only neutrons of a specific velocity can pass. For the elimination of harmonics and further "rectification" or resolution of the beam, disk choppers are provided. The disks consist of neutron-absorbing material and are provided with windows. Usually a plurality of such disks are arranged in series. Since the more drums and disks which are arranged in series for a given neutron beam, the greater will be the sharpness of the resolution and the manner and energetic character of the resulting beam, it is desirable to provide numerous disks driven absolutely synchronously with a given angular offset of the disks.
To this end it is a common practice to make use of synchronous motors driven by a common frequency generator, usually a quartz-controlled oscillator.
In such systems it is important to determine the relative angular positions of the disks of two such chopper motors and hence the relative angular positions of the magnetic-pole rotors of these synchronous motors.
It is known, in this connection, to provide a start-stop pulse counter which responds to pulses generated by an optoelectronic device connected to the pole rotor of each motor. The counter is triggered by a pulse from the frequency generator and stopped when a zero mark of the pole rotor triggers its optoelectronic device. Between the start and stop signals, a number of pulses are counted corresponding to the subdivisions or traces carried by the rotor. The relative angular positions of the rotors of the two synchronous motors is the difference between the two counted pulse trains.
The disadvantage of this system is that the precision of the measurement, which depends upon the markings carried by the rotors, is limited at higher angular velocities of the synchronous motors.